only if 意思是"只有......才......"。only if即可并列在一起，也可分开。 1. A student is allowed to enter this room only if a teacher has given permission. 只有得到教师的许可学生才可以进入这个房间。 2. Seat belts are effective only if they are correctly adjusted. 座椅安全带只有调节好才能起作用。 3. We can only reach agreement if we both put our cards on the table. 我们双方只有开诚布公来谈才能达成协议。 A: My shirt needs ironing. Could you please help me with that, Mary? 我的衬衫需要熨一下。你能帮个忙吗，玛丽？ B: No way. Do it yourself. 不行。你自己干吧。 A: Please, I promise this is the last time. 求求你啦，我保证这是最后一次。 B: OK, I'll iron your shirt for you, but only if you make me a cup of coffee. 好吧，我来为你熨衬衫，但条件是你给我煮一杯咖啡。
